FIELD OF THE INVENTION
[0001] The present invention relates to the use of a duplex stainless steel in highly corrosive
urea environments containing ammonia carbamate at high temperatures and pressures.
[0002] The invention thus also relates to the use of a duplex stainless steel in a urea
plant (i.e. plant for production of urea), and specifically in an apparatus, equipment
or device (or a part thereof) which is exposed to concentrated ammonium carbamate
at high temperature.
[0003] The invention also relates to an apparatus, equipment or device of a urea production
plant or used in a urea production process, comprising at least a part made of a corrosion
resistant duplex stainless steel.
[0004] The invention also relates to a plant and a process for the production of urea comprising
at least one apparatus, equipment or device having at least a part made of a duplex
stainless steel, and to a method of revamping an existing urea production plant by
replacing at least a part of an apparatus, equipment or device of the plant with a
part made of a duplex stainless steel.
DESCRIPTION OF PRIOR ART
[0005] Duplex stainless steels are a family of stainless steels characterized by a two-phase
microstructure consisting of grains of austenite and ferrite in roughly equal proportions.
[0006] The austenite-ferrite structure gives this family of stainless steels a combination
of favorable properties, in particular good mechanical strength and excellent resistance
to corrosion.
[0007] However, commonly available grades of duplex stainless steels, even if generally
exhibiting a good corrosion resistance, are not suitable for use under very severe
conditions, such as in a urea production plant and specifically in a high pressure
section of a urea plant.
[0008] As it is known, urea production is based on a high-temperature, high-pressure reaction
of carbon dioxide and ammonia to form ammonium carbamate, and a subsequent dehydration
reaction of the ammonium carbamate to form urea and water.
[0009] In a typical urea production plant (urea plant), these processes are generally carried
out in a urea synthesis reactor operating at high pressure and high temperature; the
aqueous urea solution produced in the synthesis reactor is then progressively concentrated,
with recover of unconverted reagents, in one or more recovery sections, for example
in a high-pressure section, a medium-pressure section and a low-pressure section;
finally, the urea is solidified in a finishing section, which normally includes a
granulator or a prilling tower.
[0010] Industrial level processes and plants for the production of urea are today largely
based on stripping processes: the synthesis solution exiting from the reactor is subjected
to heating at high pressure (substantially the same pressure of the reactor) and the
ammonium carbamate decomposes into ammonia and carbon dioxide in the liquid phase;
part of the ammonia, together with carbon dioxide, passes from the liquid phase to
the gas phase. The gas phase collected from the stripper is condensed and recycled
to the reactor.
[0011] In some industrial processes, ammonia is used as a stripping agent (ammonia-stripping
process), or the stripping is performed only by supplying heat, without any stripping
agent (self-stripping process, or thermal stripping process).
[0012] In other industrial processes, such as the so-called CO2-stripping process, the stripping
agent is gaseous carbon dioxide.
[0013] In a urea synthesis plant operating according to the ammonia-stripping process or
the self-stripping process, corrosion resistance is an essential feature.
[0014] In particular, the ammonia-stripping process and the self-stripping process have
a high pressure section, basically comprising the urea synthesis reactor and the urea
stripper (as well as auxiliary equipment and devices), where the corrosion resistance
is most important, due to the presence of the intermediate compound ammonium carbamate
solution.
[0015] The ammonia-stripping process and the self-stripping process are in fact preferably
performed at a maximum temperature of 185°C or higher (more preferably at 190°C or
higher, in particular at 205°C or higher and preferably in the range 205-215°C); at
a maximum pressure of 150 bar or higher (preferably of 156 bar or higher and more
preferably of about 160 bar or higher); and with a NH3/CO2 molar ratio (so-called
N/C ratio) in the range 3.2-3.6.
[0016] For example, stripping processes of the type described just above, operating at such
conditions, are used in the so-called "Snamprogetti Urea Technology", which is well
known to the skilled person being widely used worldwide and often cited in technical
texts and papers.
[0017] Thus, at least some apparatus, equipment or device of the urea plant, in particular
of the high pressure section thereof, such as (but not only) the urea stripper, operate
under processing conditions which are highly corrosive, particularly due to the presence
of a hot and concentrated carbamate solution at high temperatures (185°-205°C and
over) and pressures (150 bar or higher).
[0018] Similar problems are however also present in other kinds of urea production plants
also having a high pressure section.
[0019] Therefore, the high pressure section of a urea plant (in particular, but not only,
in a urea plant operating according to the ammonia-stripping process or the self-stripping
process) usually requires addition of a certain amount of oxygen (typically in form
of a stream of inerts also including oxygen) for passivating the metal surfaces (especially,
but not only, if made of austenitic stainless steels). Use of oxygen in the high pressure
section can however increase the risk to originate potentially explosive mixture and
therefore there is a concern in terms of safety.
[0020] In order to reduce the use of passivation gas streams and/or to improve corrosion
resistance, duplex stainless steels have been proposed for use in urea production
plants.
[0021] For example,
WO95/00674 discloses the use of a particular duplex stainless steel, the so called super duplex
stainless steel sold under the trademark Safurex®, for making some equipment of urea
plants.
[0022] However, the super duplex stainless steels of
WO95/00674, when used in a carbamate environment, may be not fully effective at very high temperatures
(higher than 180-200°C), such as common operation temperatures of ammonia-stripping
or self-stripping processes. Use of known duplex stainless steels is thus confined
to CO2-stripping processes.
[0023] WO2014/180761 discloses a shell-and-tube urea stripper, to be specifically used in an ammonia-stripping
or self-stripping process, having a bundle of tubes made of certain duplex stainless
steels, namely of the Safurex® steel 29Cr-6.5Ni-2Mo-N (ASME Code 2295-3 and UNS S32906),
or the DP28W™ steel 27Cr-7.6Ni-lMo-2.3W-N (ASME Code 2496-1 and UNS S32808) .
[0025] It can be appreciated that all the prior art documents cited above disclose duplex
stainless steels which do not contain cobalt.
[0026] WO2006/049572 discloses a duplex stainless steel alloy which contains also cobalt and shows high
strength, good corrosion resistance, good workability and which is weldable. The proposed
alloys are intended for use in the onshore and offshore sectors of the oil and gas
industry, while uses under more severe corrosive conditions (such as in a urea plant/process)
are not mentioned.
[0027] US2015/050180A1 discloses a duplex ferritic austenitic stainless steel containing cobalt and which
chemical composition and microstructure are favorable for use in chemical industries
wherein good uniform corrosion resistance and high strength are required, such as
for example in urea manufacturing.
[0028] Therefore, even if duplex stainless steels are known which have good corrosion resistance
and are allegedly suitable for use also in a urea production plant, there is still
a need for other, possibly more corrosion resistant duplex stainless steels which
are suitable for use in any urea environments, i.e. in any kind of urea production
plants/processes, and specifically in an apparatus operated at high temperatures in
contact with very corrosive fluids (containing ammonium carbamate) and also under
oxygen-free conditions, such as for instance (but not only) the high pressure strippers
(operated at pressure of 150 bar and more) used in an ammonia-stripping process or
a self-stripping process.
DISCLOSURE OF THE INVENTION
[0029] Accordingly, it is an object of the present invention to provide a duplex stainless
steel suitable for overcoming the problem foregoing described of the prior art.
[0030] In particular, it is an object of the present invention to provide duplex stainless
steels which are specifically and fully suitable to be used in a urea environment,
i.e. in contact with a fluid comprising ammonium carbamate, such as a concentrated
ammonium carbamate solution, and also at temperatures of at least 185°C, preferably
of at least 190°C and more preferably of 205°C and more, even under oxygen-free conditions.
[0031] It is also a specific object of the invention to provide corrosion resistant duplex
stainless steels which are suitable for use in any urea environments, i.e. in any
kind of urea production plants/processes, and specifically in an apparatus (such as
a high pressure stripper) used in an ammonia-stripping process or a self-stripping
process, and thus operating at a maximum temperature of 185°C or higher (preferably
at 190°C or higher, in particular at 205°C or higher and preferably in the range 205-215°C);
and/or at a maximum pressure of 150 bar or higher (preferably of 156 bar or higher
and more preferably of about 160 bar or higher); and/or with a NH3/CO2 molar ratio
(so-called N/C ratio) in the range 3.2-3.6.
[0032] The present invention accordingly relates to a duplex stainless steel for use in
a urea production plant and/or in a urea production process, as defined in Claim 1.
[0033] The invention also relates to an apparatus, equipment or device, in particular of
a urea production plant or used in a urea production process, comprising at least
a part made of a corrosion resistant duplex stainless steel, as defined in Claim 21.
[0034] The invention also relates to a plant and a process for the production of urea comprising
at least one such apparatus, equipment or device having at least a part made of a
duplex stainless steel as defined in Claims 22 and 23 respectively; and to a method
of revamping an existing urea production plant by replacing at least a part of an
apparatus, equipment or device of the plant with a part made of a duplex stainless
steel as defined in Claim 24.
[0035] Advantageous preferred features of the invention are the subject matter of the dependent
claims.
[0036] The duplex stainless steels of the invention are specifically characterized by a
combination of Ni, Co and Mo: in fact, it has been recognized that such three elements,
used together according to specific composition rules, have an unexpected combined
effect on the corrosion resistance as well as on other favorable material properties.
[0037] In fact, it has been found that these three elements (Ni, Co, Mo) effectively increase
corrosion resistance of a duplex stainless steel (having the particular composition
of the invention) if each element is used in a specific content range and the contents
of the three elements are linked to one another by a composition parameter Z which
ranges between a minimum value Z
min and a maximum value Z
max.
[0038] In particular, the duplex stainless steels of the invention have a composition parameter
Z ranging between 14,95 and 19,80.
[0039] Composition parameter Z is a parameter representative of the combined contents of
Ni, Co, Mo and defined by formula (I) :
where Ni, Co, Mo indicate the weight percentage of Ni, Co, Mo respectively.
[0040] According to the invention:
14,95 ≤ Z ≤ 19,80
[0041] In other words, the inventors have found that duplex stainless steels having the
particular compositions of the invention also exhibit an excellent corrosion resistance
(in particular, in urea environments) if parameter Z is maintained in the ranges defined
above, i.e. if components Ni, Co and Mo are used in amounts which satisfy formula
(II):
where:
Ni, Co, Mo indicate the weight percentage of Ni, Co, Mo respectively
Zmin = 14,95
Zmax = 19,80
[0042] Experimental test confirm that duplex stainless steels according to the invention,
i.e. having a combined content of Ni, Co and Mo as previously defined, satisfying
formula (II), have a corrosion rate in urea environments (containing ammonium carbamate)
significantly lower than prior art materials, even at high temperature/pressure and
in oxygen-free conditions.
[0043] Such a result cannot be expected in view of the prior art teachings.
[0044] It is in fact commonly recognized in the art (as reported by several scientific papers)
that the content of nickel (Ni) in austenitic steels is detrimental under low oxygen
condition.
[0045] Therefore, it is commonly understood that corrosion resistance of duplex stainless
steels take advantage from a low content of nickel.
[0046] On the contrary, the inventors of the present invention have recognized that a certain
amount of nickel, lower than in usual austenitic steels but higher than a minimum
threshold, has indeed a good impact on corrosion resistance of a duplex stainless
steel, if nickel is associated with cobalt (Co) and molybdenum (Mo) according to specific
rules.
[0047] Specifically, the duplex stainless steels of the invention have a content of nickel
ranging between 5,5% and 8%, preferably from 6,0% to 7,5% (here and below, all percentages
are intended, if not otherwise specified, as weight percentages with respect to the
total weight of the steel).
[0048] Nickel is in fact an austenite forming element and a certain amount of nickel is
needed to maintain an equilibrium between ferrite and austenite phases. From the other
hand, nickel has a negative impact on intermetallic precipitation.
[0049] According to the invention, cobalt is used in combination with nickel (and replacing
part of the nickel) to obtain the required balance between ferrite and austenite phases
and to improve corrosion resistance.
[0050] The inventors of the present invention have in fact realized that the content of
nickel can be reduced by replacing nickel with cobalt, that works as a partial substitute
and surprisingly also has the additional advantage of improving the corrosion resistance
of the duplex stainless steels having the particular compositions of the invention.
[0051] Cobalt, in fact (unlike nickel), reduces the precipitations of intermetallic phases,
strengthens the ferrite matrix and has a positive effect as austenite forming element.
[0052] Specifically, the duplex stainless steels of the invention have a content of Co in
the range between 0.01% and 0,8%, preferably from 0,01% to 0,6%, more preferably from
0,02 to 0,6%, in particular from 0,04% to 0,6%.
[0053] According to the invention, the contents of nickel and cobalt is also linked to the
content of molybdenum.
[0054] Molybdenum is a ferrite forming element which accelerates the precipitation of intermetallic
phases especially in the presence of high levels of chromium (such as in the duplex
stainless steels of the invention); therefore, the content of molybdenum should not
exceed a maximum threshold.
[0055] On the other hand, a certain amount of molybdenum is beneficial for ammonium carbamate
corrosion resistance and localised corrosion resistance, especially in the presence
of ammonium carbamate and under oxygen-free conditions.
[0056] Specifically, molybdenum is in the range between 2% and 2,5%.
[0057] The features of the invention as previously defined also provides a method to design
a duplex stainless steel for use in very corrosive environments, in particular in
a urea plant/process.
[0058] In particular, the invention provides the rules for selecting an effective content
of Ni, Co, Mo.
[0059] Once selected the content/amount of two out of the three components (Ni, Co, Mo),
for example by taking into account the above technical considerations about expected
effects of each individual elements, the content/amount of the third component is
calculated by applying the relationships of the invention.
[0060] In addition to Ni, Co and Mo, the duplex stainless steels of the invention have a
relatively high content of chromium (Cr), which increases corrosion resistance in
ammonium carbamate solution environments, and at the same time allows a good microstructure
without precipitation of third phases and a good hot workability.
[0061] Chromium has in fact a beneficial effect on corrosion resistance and allows higher
process temperatures in urea production applications. Chromium is also beneficial
for other types of corrosion such as pitting or crevice. On the other hand, high amounts
of chromium increase the possibility of precipitation of intermetallic phases and
are detrimental to hot workability. Therefore, the amount of chromium is higher than
30% but lower than 35%, preferably ranging between 30,5 and 35%, more preferably between
30,5 and 33%, even more preferably between 30,5 and 32%, in particular between 30,5
and 31,6%.
[0062] The duplex stainless steel of the invention may also contain the following elements:
Carbon (C). Carbon generally improves mechanical strength; however, according to the
invention high contents of carbon are avoided in order to prevent precipitation of
carbides. Therefore, the amount of carbon is not higher than 0,03%, preferably from
0,001% to 0,02%.
Silicon (Si). Silicon is used as a ferrite forming element and for deoxidization in
the steel mill, i.e. in the manufacturing process of the duplex stainless steels.
High amounts of silicon are avoided in order to reduce the possibility of precipitation
of intermetallic phases. Thus the amount of silicon is not higher than 0,5%, preferably
from 0,001% to 0,5%.
Manganese (Mn). Manganese increases the solubility of nitrogen (N), but has also a
negative impact on corrosion resistance. Therefore, the amount of manganese is not
higher than 2,5%, preferably from 0,5% to 2,2%, in particular from 1,0% to 2,2%.
Tungsten (W). Tungsten is a ferrite forming element. Tungsten also enhances general
corrosion resistance. In particular, in the same way as Cr, Mo and N, also W increases
pitting and crevice resistance. However, W accelerates the precipitation of intermetallic
phases so its content is maintained below 2,5%, preferably from 0,001% to 2,5%, more
preferably from 0,02% to 1%.
Nitrogen (N). Nitrogen is an austenite forming element. Nitrogen also enhances the
microstructure stability delaying the precipitation of intermetallic phases and increases
the strength of the metal matrix. Nitrogen is added also to increase the pitting and
crevice corrosion resistance. For these reasons, at least 0,3% of nitrogen is used.
On the other hand, higher contents of nitrogen would lead to poor hot workability,
therefore the maximum value of N content is 0,6%. Thus, the content of N ranges from
0,3 to 0,6%, preferably from 0,35% to 0,6%, in particular from 0,4% to 0,6%.
Copper (Cu). Copper has in general a positive effect depressing the intermetallic
precipitation kinetics, especially when relatively high amounts of Mo and W are present.
However, for urea production applications copper is a harmful element because it forms
complex ions with ammonia and deteriorates corrosion resistance. Therefore, Cu content
is limited to a maximum of 1%, preferably from 0,001% to 0,9%, more preferably from
0,10 to 0,90% Cu and in particular from 0,10 to 0,40%.
[0063] Since the duplex stainless steels of the invention have a relatively high content
of chromium (as well as nitrogen), hot workability could be negatively affected. In
order to facilitate processing (in particular, hot forming) of the duplex stainless
steels of the invention, one or more of the following elements are optionally added:
Calcium (Ca): 0,004% or less, preferably from 0,001% to 0,004%;
Magnesium (Mg): 0,004% or less, preferably from 0,001% to 0,004%;
One or more rare-earth elements: 0,1% or less, preferably 0,05% or less (total amount).
[0064] Preferably, the rare-earth elements are selected in the group consisting of Lanthanum
(La), Cerium (Ce), Praseodymium (Pr) and mixtures thereof.
[0065] Rare-earth elements (metals) have very high deoxidation and desulphurization capacities
and also decrease the average size of inclusions. They have a beneficial effect on
hot workability based on the ability to combine with impurities that can segregate
at grain boundaries (such as sulphur) and modify the shape and composition of the
inclusions.
[0066] The steel compositions of the invention may also include unavoidable impurities such
as Phosphorus (P) and Sulphur (S). The content of P and S should however be maintained
as low as possible. In particular, high amounts of S are detrimental to hot workability.
Thus, the S content should be less than 0,005% and the P content should be less than
0,025%. Typical amounts are less than 0,0005% for S and less than 0,020% for P.
[0067] The ferrite content of the duplex steel (austeno-ferritic alloy) according to the
present invention is also of some importance for the corrosion resistance. According
to some embodiments, therefore, the ferrite content ranges from 30% to 70% by volume,
preferably from 35 to 60%vol., more preferably from 40 to 60%vol. The duplex stainless
steels of the invention are suitably resistant to corrosion even when exposed to ammonium
carbamate at high pressure (in particular, at a maximum pressure of 150 bar and higher,
preferably of 156 bar and higher, more preferably of 160 bar and higher) and high
temperature (in particular 185°C and higher, preferably 190°C and higher, more preferably
205°C and higher), and even in oxygen-free condition.
[0068] The invention thus provides improved formulations of duplex stainless steels, fully
suitable for use in very corrosive conditions such as in a urea environment, i.e.
in contact with a fluid comprising ammonium carbamate, also at temperatures of 185°C
and more (and even at 205°C and more) and even under oxygen-free conditions.
[0069] In particular, the duplex stainless steels of the invention are intended for use
in contact with ammonium carbamate solutions having a concentration of ammonium carbamate
ranging from 15%w to 95%w, in particular from 50%w to 95%w; and/or at a temperature
of 185°C or more, in particular of 190°C or more, in particular of 205°C or more).
[0070] The highly corrosion resistant duplex stainless steels of the invention are suitable
for use in any urea environments, i.e. in any kind of urea production plants/processes,
and specifically in apparatuses operated at high temperatures (185°C, 190°C but also
205°C and higher) in contact with fluids containing ammonium carbamate and also under
oxygen-free conditions, such as for instance (but not only) the high pressure strippers
used in the ammonia-stripping process or the self-stripping process.
[0071] Thus, the duplex stainless steels of the invention are especially useful for manufacturing
equipment and devices (or parts thereof) which are exposed to concentrated ammonium
carbamate at high temperature, such as parts of the heat exchanger tubes and/or, or
for example, tubes of strippers.
[0072] The duplex stainless steels of the invention exhibit an excellent corrosion resistance
in carbamate solutions (even in oxygen-free condition) also at temperature of 205°C
and higher.
[0073] The materials of the invention are therefore suitable to be used in a urea production
plant of any kinds, including in particular the most demanding conditions of an ammonia-stripping
or self-stripping process.
[0074] The invention thus relates to the use of the duplex stainless steel as disclosed
herein in a urea production plant, and specifically in an apparatus, equipment or
device (or a part thereof) which is exposed to concentrated ammonium carbamate at
high temperature.
[0075] The invention also relates to an apparatus, equipment or device, in particular of
a urea production plant or used in a urea production process, comprising at least
a part made of a corrosion resistant duplex stainless steel as disclosed herein.
[0076] The invention also relates to a plant and a process for the production of urea comprising
at least one apparatus, equipment or device having at least a part made of a duplex
stainless steel as disclosed herein; and to a method of revamping an existing urea
production plant by replacing at least a part of an apparatus, equipment or device
of the plant with a part made of a duplex stainless steel as disclosed herein.
[0077] As a result of the specific compositions of the duplex stainless steels of the invention,
the following additional advantages are also achieved over the prior art, in particular
in case of use in a high pressure apparatus of the urea plant:
- the corrosion rate in an piece of equipment (apparatus/device or part thereof) made
of the duplex stainless steels of the invention drastically decreases with respect
to a piece of equipment made of prior art materials;
- the need of passivation air is drastically reduced or even eliminated;
- the thickness of the apparatus/device, in particular of the high pressure piping loop,
can be reduced, thus resulting in a significant reduction of the total weight and
cost of the high pressure section, since the duplex stainless steels of the invention
also have high mechanical characteristics;
- the temperature at the bottom of the stripper can be increased without increasing
the corrosion rate;
- it is possible to avoid using, for the high pressure equipment, different materials
with different features and prescription in terms of material specifications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0078] In the appended drawings:
- figure 1 contains a table (Table 1) reporting the composition of exemplary samples
of duplex stainless steels according to the invention, as well as of some reference
samples;
- figure 2 contains a table (Table 2) reporting the results of corrosion resistance
tests performed on the samples of Table 1.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0079] Duplex stainless steels according to the invention contains in weight % (%w):
C max 0,03
Si max 0,5
Mn max 2,5
Cr from more than 30,0 to 35,0
Ni 5, to 8,0
Co 0,01 to 0,8
Mo 2,0 to 2,5
W max 2,5
N 0,3-0,6
Cu max 1,0
Ca max 0,0040
Mg max 0,0040
one or more rare-earth elements max 0,1
the balance being Fe and impurities (as commonly understood, impurities are all those
elements and compounds which are not purposively added to the steel formulation, but
are however present in small amounts being contained in the raw materials used for
manufacturing the duplex stainless steel).
[0080] The duplex stainless steels of the invention are further characterized in that the
content of Ni, Co, Mo is such that:
where:
Ni, Co, Mo indicate the weight percentage of Ni, Co, Mo respectively;
Zmin = 14,95;
Zmax = 19,80.
[0081] In other words, the duplex stainless steels of the invention have a composition parameter
Z, representative of the combined contents of Ni, Co, Mo and defined by formula (I)
:
where Ni, Co, Mo indicate the weight percentage of Ni, Co, Mo respectively;
and wherein
14,95 ≤ Z ≤ 19,80.
Examples
[0082] Exemplary steel compositions according to the invention comprise, in percentages
by weight:
C: 0,03% or less;
Si: 0,5% or less;
Mn: 2,5% or less;
Cr: 30,5% to 35%;
Ni: 5,5% to 8%;
Mo: 2% to 2,5%;
W: 0,02% to 1,0%;
Co: 0,01% to 0,8%;
N: 0,3% to 0,6%;
Cu: 1% or less;
one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
one or more rare earth elements in a total amount of 0,05% or less;
the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062∗(Ni + Co) + 4,185∗Mo is between 14,95 and 19,80.
[0083] Other embodiments of the steel of the invention comprise, in percentages by weight:
C: 0,001% to 0,03%;
Si: 0,001% to 0,5%;
Mn: 0,001% to 2,5%;
Cr: more than 30% to 35%;
Ni: 5,5% to 8%;
Mo: 2% to 2,5%;
W: 0,4% to 0,8%;
Co: 0,01% to 0,8%;
N: 0,3% to 0,6%;
Cu: 0,001% to 1%;
one or more of the following:
Ca: 0,001% to 0,004%;
Mg: 0,001% to 0,004%;
one or more rare earth elements in a total amount of 0,001% to 0,1%;
the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062∗(Ni + Co) + 4,185∗Mo is between 14,95 and 19,80.
[0084] Other compositions according to the invention comprise, in percentages by weight:
C: 0,001% to 0,03%;
Si: 0,5% or less;
Mn: 0,5% to 2,2%;
Cr: 30,5% to 34%;
Ni: 5,5% to 8%;
Mo: 2% to 2,5%;
W: 2,5% or less;
Co: 0,01% to 0,8%;
N: 0,3% to 0,6%;
Cu: 1% or less;
one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
one or more rare earth elements in a total amount of 0,05% or less;
the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062∗(Ni + Co) + 4,185∗Mo is between 14,95 and 19,80.
[0085] Yet other compositions according to the invention comprises, in percentages by weight:
C: 0,02% or less;
Si: 0,001% to 0,5%;
Mn: 2,5% or less;
Cr: 30,5% to 32%;
Ni: 5,5% to 8%;
Mo: 2% to 2,5%;
W: 0,1% to 1%;
Co: 0,01% to 0,8%;
N: 0,3% to 0,6%;
Cu: 0,15% to 0,25%;
having one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
La, Ce, Pr or other rare earth elements: 0,05% or less
the remainder being Fe and unavoidable impurities;
satisfying the relationship CRC = 1,062∗(Ni + Co) + 4,185∗Mo is between 14,95 to 19,80.
[0086] Other compositions according to the invention comprises, in percentages by weight:
C: 0,03% or less;
Si: 0,5% or less;
Mn: 0,001% to 2,2%;
Cr: 31% to 35%;
Ni: 6% to 7,5%;
Mo: 2% to 2,5%;
W: 2,5% or less;
Co: 0,01% to 0,8%;
N: 0,4% to 0,6%;
Cu: 0,9% or less;
one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
one or more rare earth elements in a total amount of 0,05% or less;
the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062∗(Ni + Co) + 4,185∗Mo is between 14,95 and 19,80.
[0087] Other examples according to this invention comprise, in percentages by weight:
C: 0,03% or less;
Si: 0,5% or less;
Mn: 0,5% to 2,2%;
Cr: 30,5% to 35%;
Ni: 5,5% to 6,5%;
Mo: 2% to 2,5%;
W: 0,001% to 2,5%;
Co: 0,01% to 0,6%;
N: 0,35% to 0,6%;
Cu: 1% or less;
one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
one rare earth element selected from La, Ce, Pr or a combination thereof: 0,05% or
less
the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062∗(Ni + Co) + 4,185∗Mo is between 14,95 and 19,80.
[0088] For example, the present invention relates to elementary steel compositions that
comprise, in percentages by weight:
C: 0,03% or less;
Si: 0,5% or less;
Mn: 2,2% or less;
Cr: 31% to 32%;
Ni: 5,5% to 8%;
Mo: 2% to 2,5%;
W: 2,5% or less;
Co: 0,02% to 0,4%;
N: 0,3% to 0,6%;
Cu: 0,001% to 1%;
one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
one rare earth element selected from La, Ce, Pr or a combination thereof: 0,05% or
less
the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062∗(Ni + Co) + 4,185∗Mo is between 14,95 and 19,80.
[0089] Other exemplifying composition according to the invention comprise, in percentages
by weight:
C: 0,03% or less;
Si: 0,5% or less;
Mn: 2% or less;
Cr: 30,5% to 33%;
Ni: 5,5% to 8%;
Mo: 2% to 2,5%;
W: 0,2% to 1%;
Co: 0,02% to 0,4%;
N: 0,3% to 0,6%;
Cu: 1% or less;
one or more of the following:
Ca: 0,001% to 0,004%;
Mg: 0,001% to 0,004%;
La, Ce, Pr or other rare earth elements: 0,001% to 0,05%
the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062∗(Ni + Co) + 4,185∗Mo is between 14,95 and 19,80.
[0090] Further example compositions according to the invention comprise, in percentages
by weight:
C: 0,02% or less;
Si: 0,5% or less;
Mn: 0,5% to 2,2%;
Cr: 30,5% to 34%;
Ni: 5,5 to 8%;
Mo: 2 to 2,5%;
W: 0,02 to 1%;
Co: 0,02 to 0,6%;
N: 0,3 to 0,6%;
Cu: 0,20% to 0,9%;
one or more of the following:
Ca: 0,004% or less;
Mg: 0,004% or less;
one or more rare earth elements in a total amount of 0,05% or less;
the remainder being Fe and unavoidable impurities;
satisfying the relationship: Z = 1,062∗(Ni + Co) + 4,185∗Mo is between 14,95 and 19,80.
[0091] In particular, duplex stainless steels having the compositions in Table 1 were prepared
and tested (in Table 1, some components are not indicated, being however in the amounts
as previously disclosed).
[0092] The samples were prepared as common in the field and tested according to standard
testing procedure.
[0093] In particular, corrosion tests were performed in a high pressure autoclave in ammonium
carbamate solution at high pressure and high temperature (conditions representative
of typical operation conditions in urea plants, in particular in the tubes of a urea
stripper).
[0094] In particular, the corrosion resistance of the duplex stainless steels of the invention
was tested in an oxygen-free carbamate solution, having a composition simulating the
worst conditions normally occurring in the tubes of a high pressure section urea stripper
of a urea plant, and at a temperature of 208°C.
[0095] In more detail: the corrosion behavior of the laboratory heats was checked via immersion
tests that were conducted in a 5-liter Zirconium autoclave. The autoclave was equipped
with adequate feed and discharge lines and a stirrer. The test solution contained
a mixture of urea, ammonia and water, at concentrations similar to those of the urea
synthesis process. Temperature and pressure for the experiments were set in the upper
level of the typical ranges measured in a urea stripper, 180-210°C and 140-200 bar,
respectively. The test solution was degassed before starting the tests to eliminate
oxygen from the system. These experiments were designed to simulate the most severe
conditions in a stripper of a urea plant without oxygen injection; note that under
current working conditions in a urea plant, the stainless steel would perform even
better, due to the presence of low amounts of oxygen and less aggressive conditions.
[0096] Test duration was 13 and 30 days. ASTM G31 (Standard Practice for Laboratory Immersion
Corrosion Testing of Metals) standard indications were followed for test specimen
preparation and the corrosion rate was measured by the gravimetric method.
[0097] After exposures of 13 days and 30 days respectively in the oxygen-free carbamate
solution, the corrosion resistance was evaluated by calculating the corrosion rate
(expressed in mm/year).
[0098] The results are shown in Table 2.
[0099] The results confirm that the samples (A1-A5) made of a duplex stainless steel according
to the invention, i.e. satisfying the composition requirements of the invention (in
particular with respect to the combined content of Ni, Co, Mo), have a corrosion rate
significantly lower than comparative samples Ref1, Ref2, Ref3 and thus a better corrosion
resistance.
[0100] In fact, the experimental tests confirm that when Z satisfies the requirement: 14.95
≤ Z ≤ 19.80, corrosion values are significantly lower than those exhibited by reference
materials.
[0101] Corrosion values would be even significantly lower in working conditions in a urea
plant, since the experimental set-up conditions are much more aggressive.
[0102] Finally, although the invention has been disclosed in relation to the above-mentioned
preferred embodiments, it is to be understood that many other possible modifications
and variations can be made without departing from the scope of the appended claims.
1. Use of a duplex stainless steel in a urea production plant and/or in a urea production
process, wherein the duplex stainless steel is used in a urea environment and in contact
with a fluid comprising ammonium carbamate, and wherein the duplex stainless steel
contains in weight percentage (%w):
C 0,03 or less
Si 0,5 or less
Mn 2,5 or less
Cr from more than 30,0 to 35,0
Ni from 5,5 to 8,0
Co from 0,01 to 0,8
Mo from 2,0 to 2,5
W 2,5 or less
N from 0,3 to 0,6
Cu 1,0 or less
having one or more of the following:
Ca 0,0040 or less
Mg 0,0040 or less
one or more rare-earth elements in a total amount of 0,1 or less;
the balance being Fe and impurities;
wherein the duplex stainless steel has a composition parameter (Z), representative
of the combined contents of Ni, Co, Mo and defined by formula (I):
where Ni, Co, Mo indicate the weight percentage of Ni, Co, Mo respectively;
said composition parameter (Z) ranging between 14,95 and 19,80.
2. The use according to claim 1, wherein the duplex stainless steel contains 30,5-35%w
Cr.
3. The use according to one of the preceding claims, wherein the duplex stainless steel
contains 30,5-32%w Cr, more preferably 30,5-31,6%w Cr.
4. The use according to one of the preceding claims, wherein the duplex stainless steel
contains 0,10-0,90%w Cu or 0,10-0,40%w Cu.
5. The use according to one of the preceding claims, wherein the duplex stainless steel
contains 0,02-0,6%w Co.
6. The use according to one of the preceding claims, wherein the duplex stainless steel
contains 6,0-7,5%w Ni.
7. The use according to one of the preceding claims, wherein the duplex stainless steel
contains Manganese from 0,5 to 2,2%w.
8. The use according to one of the preceding claims, wherein the duplex stainless steel
contains Tungsten from 0,02 to 1,0%w.
9. The use according to one of the preceding claims, wherein the duplex stainless steel
contains Calcium from 0,001 to 0,004%w.
10. The use according to one of the preceding claims, wherein the duplex stainless steel
contains Magnesium (Mg) from 0,001 to 0,004%w.
11. The use according to one of the preceding claims, wherein the duplex stainless steel
contains one or more rare-earth elements in a total amount of 0,05 or less.
12. The use according to one of the preceding claims, wherein the duplex stainless steel
contains one or more rare-earth elements selected in the group consisting of Lanthanum
(La), Cerium (Ce), Praseodymium (Pr) and mixtures thereof.
13. The use according to one of the preceding claims, wherein the duplex stainless steel
contains, as impurities, no more than 0,025%w Phosphorus (P) and/or no more than 0,005%w
Sulphur (S).
14. The use according to one of the preceding claims, wherein the duplex stainless steel
is in contact with an ammonium carbamate solution having a concentration of ammonium
carbamate ranging from 15%w to 95%w, in particular from 50%w to 95%w.
15. The use according to one of the preceding claims, wherein the duplex stainless steel
is in contact with an ammonium carbamate solution at a temperature of 185°C or higher,
preferably of 190°C or higher, more preferably of 205°C or higher.
16. The use according to one of the preceding claims, wherein the duplex stainless steel
is used at a maximum temperature of 185°C or higher, preferably of 190°C or higher,
more preferably of 205°C or higher, more preferably in the range 205-215°C; and/or
at a maximum pressure of 150 bar or higher, preferably of 156 bar or higher and more
preferably of 160 bar or higher; and/or in an environment having a NH3/CO2 molar ratio
in the range 3,2-3,6.
17. The use according to one of the preceding claims, wherein the urea environment is
under oxygen-free conditions.
18. The use according to one of the preceding claims, wherein the duplex stainless steel
is used in a high pressure section of a urea plant.
19. The use according to one of the preceding claims, wherein the duplex stainless steel
is used in an apparatus, equipment or device performing an ammonia-stripping process
or a self-stripping process in a urea production process or plant.
20. The use according to one of the preceding claims, wherein the duplex stainless steel
is used in a high pressure stripper configured for ammonia-stripping or self-stripping
in a urea production process or plant.
21. An apparatus, equipment or device of a urea production plant or used in a urea production
process, comprising at least a part which is exposed to concentrated ammonium carbamate
at high temperature and is made of a corrosion resistant duplex stainless steel, wherein
said duplex stainless steel contains in weight percentage (%w):
C 0,03 or less
Si 0,5 or less
Mn 2,5 or less
Cr from more than 30,0 to 35,0
Ni from 5,5 to 8,0
Co from 0,01 to 0,8
Mo from 2,0 to 2,5
W 2,5 or less
N from 0,3 to 0,6
Cu 1,0 or less
having one or more of the following:
Ca 0,0040 or less
Mg 0,0040 or less
one or more rare-earth elements in a total amount of 0,1 or less;
the balance being Fe and impurities;
wherein the duplex stainless steel has a composition parameter (Z), representative
of the combined contents of Ni, Co, Mo and defined by formula (I):
where Ni, Co, Mo indicate the weight percentage of Ni, Co, Mo respectively;
said composition parameter (Z) ranging between 14,95 and 19,80.
22. A plant for the production of urea, comprising at least one apparatus, equipment or
device according to claim 21.
23. A process for the production of urea comprising at least one step performed in an
apparatus, equipment or device according to claim 21.
24. A method of revamping an existing urea production plant by replacing at least a part
of an apparatus, equipment or device of the plant with a part made by the use according
to one of the claims from 1 to 20.
1. Verwendung eines Duplex-Edelstahls in einer Harnstoff-Herstellungsanlage und/oder
in einem Harnstoff-Herstellungsprozess, wobei der Duplex-Edelstahl in einer Harnstoffumgebung
und in Kontakt mit einem Fluid verwendet wird, das Ammoniumcarbamat umfasst, und wobei
der Duplex-Edelstahl in Gewichtsprozent (Gew.-%) enthält:
C 0,03 oder weniger
Si 0,5 oder weniger
Mn 2,5 oder weniger
Cr von mehr als 30,0 bis 35,0
Ni von 5,5 bis 8,0
Co von 0,01 bis 0,8
Mo von 2,0 bis 2,5
W 2,5 oder weniger
N von 0,3 bis 0,6
Cu 1,0 oder weniger
und eines oder mehrere von Folgendem aufweist:
Ca 0,0040 oder weniger
Mg 0,0040 oder weniger;
ein oder mehrere Seltenerdelemente in einer Gesamtmenge von 0,1 oder weniger;
wobei der Rest Fe und Verunreinigungen sind;
wobei der Duplex-Edelstahl einen Zusammensetzungsparameter (Z) aufweist, der für die
kombinierten Gehalte an Ni, Co, Mo charakteristisch ist und durch die Formel (I) definiert
ist:
wo Ni, Co, Mo den Gewichtsprozentsatz von Ni, Co beziehungsweise Mo angeben;
der Verbindungsparameter (Z) in einem Bereich zwischen 14,95 und 19,80 liegt.
2. Verwendung nach Anspruch 1, wobei der Duplex-Edelstahl 30,5 bis 35 Gew.-% Cr enthält.
3. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl 30,5
bis 32 Gew.-% Cr, mehr zu bevorzugen 30,5 bis 31,6 Gew.-% Cr enthält.
4. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl 0,10
bis 0,90 Gew.-% Cu oder 0,10 bis 0,40 Gew.-% Cu enthält.
5. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl 0,02
bis 0,6 Gew.-% Co enthält.
6. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl 6,0
bis 7,5 Gew.-% Ni enthält.
7. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl von
0,5 bis 2,2 Gew.-% Mangan enthält.
8. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl von
0,02 bis 1,0 Gew.-% Wolfram enthält.
9. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl von
0,001 bis 0,004 Gew.-% Calcium enthält.
10. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl von
0,001 bis 0,004 Gew.-% Magnesium (Mg) enthält.
11. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl ein
oder mehrere Seltenerdelemente in einer Gesamtmenge von 0,05 oder weniger enthält.
12. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl ein
oder mehrere Seltenerdelemente enthält, die in der Gruppe ausgewählt sind, die aus
Lanthan (La), Cerium (Ce), Praseodym (Pr) und Mischungen davon besteht.
13. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl als
Verunreinigungen nicht mehr als 0,025 Gew.-% Phosphor (P) und/oder nicht mehr als
0,005 Gew.-% Schwefel (S) enthält.
14. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl mit
einer Ammoniumcarbamatlösung in Kontakt ist, die eine Konzentration an Ammoniumcarbamat
in einem Bereich von 15 Gew.-% bis 95 Gew.-%, insbesondere von 50 Gew.-%, bis 95 Gew.-%,
aufweist.
15. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl mit
einer Ammoniumcarbamatlösung bei einer Temperatur von 185 °C oder höher, vorzugsweise
von 190 °C oder höher, mehr zu bevorzugen von 205 °C oder höher in Kontakt ist.
16. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl bei
einer Höchsttemperatur von 185 °C oder höher, vorzugsweise von 190 °C oder höher,
noch mehr zu bevorzugen von 205 °C oder höher, mehr zu bevorzugen im Bereich von 205
bis 215 °C; und/oder bei einem Höchstdruck von 150 Bar oder höher, vorzugsweise von
156 Bar oder höher und mehr zu bevorzugen von 160 Bar oder höher; und/oder in einer
Umgebung verwendet wird, die ein NH3/CO2-Molverhältnis im Bereich von 3,2 bis 3,6
aufweist.
17. Verwendung nach einem der vorhergehenden Ansprüche, wobei die Harnstoffumgebung sich
unter sauerstofffreien Bedingungen befindet.
18. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl in
einem Hochdruckabschnitt einer Harnstoffanlage verwendet wird.
19. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl in
einer Einrichtung, Ausrüstung oder Vorrichtung verwendet wird, die einen Prozess zum
Strippen von Ammoniak oder einen Prozess zum Selbst-Strippen in einem/einer Harnstoff-Herstellungsprozess
oder -anlage ausführt.
20. Verwendung nach einem der vorhergehenden Ansprüche, wobei der Duplex-Edelstahl in
einem Hochdruck-Stripper verwendet wird, der zum Strippen von Ammoniak oder zum Selbst-Strippen
in einem/einer Harnstoff-Herstellungsprozess oder -anlage ausgestaltet ist.
21. Einrichtung, Ausrüstung oder Vorrichtung einer Harnstoff-Herstellungsanlage oder die
in einem Hamstoff-Herstellungsprozess verwendet wird, die mindestens einen Teil umfasst,
der konzentriertem Ammoniumcarbamat bei hoher Temperatur ausgesetzt wird und aus einem
korrosionsbeständigen Duplex-Edelstahl hergestellt ist, wobei der Duplex-Edelstahl
in Gewichtsprozent (Gew.-%) enthält:
C 0,03 oder weniger
Si 0,5 oder weniger
Mn 2,5 oder weniger
Cr von mehr als 30,0 bis 35,0
Ni von 5,5 bis 8,0
Co von 0,01 bis 0,8
Mo von 2,0 bis 2,5
W 2,5 oder weniger
N von 0,3 bis 0,6
Cu 1,0 oder weniger
und eines oder mehrere von Folgendem aufweist:
Ca 0,0040 oder weniger
Mg 0,0040 oder weniger
ein oder mehrere Seltenerdelemente in einer Gesamtmenge von 0,1 oder weniger;
wobei der Rest Fe und Verunreinigungen sind;
wobei der Duplex-Edelstahl einen Zusammensetzungsparameter (Z) aufweist, der für die
kombinierten Gehalte an Ni, Co, Mo charakteristisch ist und durch die Formel (I) definiert
ist:
wo Ni, Co, Mo den Gewichtsprozentsatz von Ni, Co beziehungsweise Mo angeben,
der Verbindungsparameter (Z) in einem Bereich zwischen 14,95 und 19,80 liegt.
22. Anlage zur Herstellung von Harnstoff, die mindestens eine Einrichtung, Ausrüstung
oder Vorrichtung nach Anspruch 21 umfasst.
23. Prozess zur Herstellung von Harnstoff, die mindestens eine Einrichtung, Ausrüstung
oder Vorrichtung nach Anspruch 21 umfasst.
24. Verfahren zum Erneuern einer bestehenden Harnstoff-Herstellungsanlage durch Ersetzen
von zumindest einem Teil einer Einrichtung, Ausrüstung oder Vorrichtung der Anlage
mit einem Teil, das durch die Verwendung nach einem der Ansprüche 1 bis 20 hergestellt
wird.
1. Utilisation d'un acier inoxydable duplex dans une usine de production d'urée et/ou
dans un processus de production d'urée, dans laquelle l'acier inoxydable duplex est
utilisé dans un environnement d'urée et en contact avec un fluide comprenant du carbamate
d'ammonium, et dans laquelle l'acier inoxydable duplex contient en pourcentage en
poids (% en poids) :
0,03 ou moins de C
0,5 ou moins de Si
2,5 ou moins de Mn
de plus de 30,0 à 35,0 de Cr
de 5,5 à 8,0 de Ni
de 0,01 à 0,8 de Co
de 2,0 à 2,5 de Mo
2,5 ou moins de W
de 0,3 à 0,6 de N
1,0 ou moins de Cu
ayant un ou plusieurs des éléments suivants :
0,0040 ou moins de Ca
0,0040 ou moins de Mg
un ou plusieurs éléments des terres rares dans une quantité totale de 0,1 ou moins
;
le reste étant du Fe et des impuretés ;
dans laquelle l'acier inoxydable duplex a un paramètre de composition (Z), représentatif
des teneurs combinées en Ni, Co, Mo et défini par la formule (I) :
où Ni, Co, Mo indiquent respectivement le pourcentage en poids de Ni, Co, Mo ;
ledit paramètre de composition (Z) allant de 14,95 à 19,80.
2. Utilisation selon la revendication 1, dans laquelle l'acier inoxydable duplex contient
30,5 à 35 % en poids de Cr.
3. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex contient 30,5 à 32 % en poids de Cr, de manière davantage préférée 30,5 à 31,6
% en poids de Cr.
4. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex contient 0,10 à 0,90 % en poids de Cu ou 0,10 à 0,40 % en poids de Cu.
5. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex contient 0,02 à 0,6 % en poids de Co.
6. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex contient 6,0 à 7,5 % en poids de Ni.
7. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex contient de 0,5 à 2,2 % en poids de manganèse.
8. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex contient de 0,02 à 1,0 % en poids de tungstène.
9. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex contient de 0,001 à 0,004 % en poids de calcium.
10. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex contient de 0,001 à 0,004 % en poids de magnésium (Mg).
11. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex contient un ou plusieurs éléments des terres rares dans une quantité totale
de 0,05 ou moins.
12. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex contient un ou plusieurs éléments des terres rares choisis dans le groupe consistant
en le lanthane (La), le cérium (Ce), le praséodyme (Pr) et des mélanges de ceux-ci.
13. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex contient, en tant qu'impuretés, pas plus de 0,025 % en poids de phosphore (P)
et/ou pas plus de 0,005 % en poids de soufre (S).
14. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex est en contact avec une solution de carbamate d'ammonium ayant une concentration
en carbamate d'ammonium allant de 15 % en poids à 95 % en poids, notamment de 50 %
en poids à 95 % en poids.
15. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex est en contact avec une solution de carbamate d'ammonium à une température
de 185 °C ou plus, de préférence de 190 °C ou plus, de manière davantage préférée
de 205 °C ou plus.
16. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex est utilisé à une température maximale de 185 °C ou plus, de préférence de
190 °C ou plus, de manière davantage préférée de 205 °C ou plus, de manière davantage
préférée dans la plage de 205 à 215 °C ; et/ou à une pression maximale de 150 bar
ou plus, de préférence de 156 bar ou plus et de manière davantage préférée de 160
bar ou plus ; et/ou dans un environnement ayant un rapport molaire NH3/CO2 dans la plage de 3,2 à 3,6.
17. Utilisation selon l'une des revendications précédentes, dans laquelle l'environnement
d'urée est dans des conditions exemptes d'oxygène.
18. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex est utilisé dans une section haute pression d'une usine d'urée.
19. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex est utilisé dans un appareil, équipement ou dispositif réalisant un processus
de stripage d'ammoniac ou un processus d'autostripage dans un processus ou une usine
de production d'urée.
20. Utilisation selon l'une des revendications précédentes, dans laquelle l'acier inoxydable
duplex est utilisé dans un dispositif de stripage haute pression configuré pour le
stripage d'ammoniac ou l'autostripage dans un processus ou une usine de production
d'urée.
21. Appareil, équipement ou dispositif d'une usine de production d'urée ou utilisé dans
un processus de production d'urée, comprenant au moins une partie qui est exposée
à du carbamate d'ammonium concentré à une température élevée et est réalisée en un
acier inoxydable duplex résistant à la corrosion, dans lequel ledit acier inoxydable
duplex contient en pourcentage en poids (% en poids) :
0,03 ou moins de C
0,5 ou moins de Si
2,5 ou moins de Mn
de plus de 30,0 à 35,0 de Cr
de 5,5 à 8,0 de Ni
de 0,01 à 0,8 de Co
de 2,0 à 2,5 de Mo
2,5 ou moins de W
de 0,3 à 0,6 de N
1,0 ou moins de Cu
ayant un ou plusieurs des éléments suivants :
0,0040 ou moins de Ca
0,0040 ou moins de Mg
un ou plusieurs éléments des terres rares dans une quantité totale de 0,1 ou moins
;
le reste étant du Fe et des impuretés ;
dans lequel l'acier inoxydable duplex a un paramètre de composition (Z), représentatif
des teneurs combinées en Ni, Co, Mo et défini par la formule (I) :
où Ni, Co, Mo indiquent respectivement le pourcentage en poids de Ni, Co, Mo ;
ledit paramètre de composition (Z) allant de 14,95 à 19,80.
22. Usine pour la production d'urée, comprenant au moins un appareil, équipement ou dispositif
selon la revendication 21.
23. Processus pour la production d'urée comprenant au moins une étape réalisée dans un
appareil, équipement ou dispositif selon la revendication 21.
24. Procédé de modernisation d'une usine de production d'urée existante en remplaçant
au moins une partie d'un appareil, équipement ou dispositif de l'usine par une partie
réalisée par l'utilisation selon l'une des revendications 1 à 20.